What is the symbol for mega?

Excuse me, but could you please clarify something for me? I'm a bit confused about the symbol used to represent the prefix "mega" in various contexts. Is it "M" or "Mg" that we typically use to denote something that is a million times the base unit? For instance, in the world of finance and cryptocurrency, we often encounter numbers that are expressed in multiples of millions or billions. So, when discussing large amounts of currency or data storage capacities, which symbol should be used to accurately convey the meaning of "mega"? Your insights would be greatly appreciated.